A mating cable harness for the 1.5mm header is made by TE Connectivity (2058943-5, Digikey A100196-ND). The radio normally has no external connector for this header, but there is a DE-sized cutout on the back below the center heat sink protrusion. Because the COS output is active low and the NHRC-2.1 repeater controller requires an active high signal, a conversion circuit is required. The inverted signal can be connected to an unused pin on the DE-9 connector, making the complete external pinout: